x/pkgsite: support @branch for branch names other than "master" and "main"

[mvdan]

Splitting off from #41312 (comment):

We're still open to a contribution for supporting non-master and non-main branch names.

That issue is originally about supporting default branches, like git's HEAD. This issue is separate; it's about supporting specific branch names that exist in VCS. Presumably this should be easier to implement, as the module proxy already supports listing the versions for any VCS branch.

cc @jba @julieqiu

[mvdan]

@julieqiu I took a look at what you did for main in golang/pkgsite@91e83ca. What kind of change would we make for arbitrary branch names? It's not clear to me, because isSupportedVersion would have to always return true for any string that could be a branch name.

I also see ScheduleFetch calls for branch queries, and I wonder if arbitrary branch names could case DoS-like attacks. For example, I could query for @b001, then @b002, @b003, etc. This kind of spamming is already possible with master and main though, and it seems like every branch request causes a scheduling, so this might not make matters worse.

[mvdan]

I've opened a sibling issue for @some-commit here: #48100